For an active laser medium to have gain, the medium must make use of an inverted population in the levels which are to interact with the signal frequency. Actually, when the term inverted is used, it does not imply that thermal populations are reversed, although that is the situation in some cases. Rather, it is intended to indicate that the upper state population is greater than the lower. Broadly speaking, there are different inversion systems which are of importance in solid state lasers.
One method of population inversion applicable to nonsemiconducting solids involves the use of three energy levels. The function of the third and highest level is solely in the inversion process. The only requirement insofar as the levels are concerned is that their retransition probabilities, that is the probability that an ion will make a transition from one state to another, for both the signal and the pump power, and their relaxation time, that is the time required for an ion to go from a high energy state to a lower energy state, be appropriate.
The active lasing medium of a solid state optically pumped laser is normally a polished crystalline or glass material which acts as host for a suitable amount (typically 0.1 mole percent) of an activator impurity element, such as ions of chromium or neodymium. When incoherent optical-pumping radiation is applied to the laser, these activator ions are excited from their ground state to higher energy states. The excited ions quickly undergo nonradiative transitions to a metastable state, from which they decay relatively slowly by radiative transitions to the terminal state, which may be an intermediate level. This fluorescent transition provides the photons for the laser. The means of decay from the terminal state to the ground state, which is the lowest state, is not important, but it should be sufficiently rapid to keep the terminal state depopulated. When the terminal state is not the same as the ground state, this is termed a four level optically pumped laser which is a more general type. Some very important lasers, for example, ruby, are three level devices, and in that case the terminal level is coincident with the ground level.
In four level lasers, population inversion is immediately achieved and optical gain is realized as soon as sufficient ions are pumped into the metastable state to overcome the optical circuit losses. In three level lasers, at least half of the ions must be pumped from the ground state into the metastable state before population inversion and gain can be realized. Thus, four level lasers require much less optical-pumping power to achieve lasing and have higher conversion efficiencies under moderate pumping levels. For these reasons, four level lasers offer a practical means of realizing CW operation in solid-state lasers.
Most activator ions in solid state optically-pumped lasers belong to the rare earth group of elements, the reason being that the electronic charge clouds of rare-earth ions are quite small. Thus, there is little overlap of the electron cloud with the adjacent host lattice ions and, consequently, little broadening of the spectral transitions. A narrow spectral width for the fundamental emission line is necessary since the pump power required to achieve lasing threshold is proportional to this line width.
The crystalline or glass laser material is usually fabricated in a rod configuration with highly reflecting surfaces forming a Fabry-Perot resonator. In early solid state lasers, the rod ends were ground accurately flat and parallel and coated with a multi-layer dielectric material or a thin layer of metal to obtain a reflectivity close to one. Since that time many different end conditions have been in common use, such as one flat end and one total internal reflecting end or both ends spherical. The threshold conditions for establishing laser oscillation in any optical resonator is reached when the gain of the optical traveling wave passing through the amplifying medium (excited activator ion) just balances the losses associated with the resonator.
In an optical cavity, which is large in all dimensions compared with the optical wavelength, many resonant modes can exist, just as for a large microwave resonator. Lasing (positive feedback) can build up only in these discreet modes, and the gain available in any particular mode is a function of how close its resonant frequency is to the center frequency of the active-ion fluorescent line. The losses in a particular mode depend on the configuration of reflectors that make up the optical cavity. The rate of stimulated emission must be related to a single mode and is equal to the spontaneous emission into a single mode times the number of photons in the mode (radiation energy density per mode). In choosing an active laser medium, the major concerns are the specific application and the desired frequency or wavelength.
Regarding the application, the need for the technology is driven by the system application. For example, the Navy is concerned with secure communications ship to shore, ship to plane and ship to ship. If a laser is utilized in the communication link, it is desirable to have good transmissivity properties through the atmosphere. The disadvantage in choosing an active medium for a particular application arises in that only certain windows, that is spectral windows, are available for satisfactory transmission. This is due to the fact that certain molecular species exist in the atmosphere that absorb the laser scatter. For example, carbon dioxide and carbon monoxide in the atmosphere exhibit a fairly heavy absorption in the three to five micron band. However, there is a small window that exists around 4.5 microns that allows adequate transmission. Transmission at other wavelengths would require the use of much higher powered transmitters to overcome the atmospheric absorption.
Even though a window may exist where communication is desirable, an active laser medium must be available with a spectral emission line at the proper wavelength. To realize a particular wavelength is somewhat of a hit and miss technique in that the available active laser mediums must be examined to see if one has adequate properties and sufficient gain at a given wavelength for utilization in an optical cavity to provide a laser.
In view of the above problems, there exists a need to further develop new pumping techniques in conjunction with various rare earth ions to determine if they will produce sufficient gain in an optical cavity. These new frequencies will expand the range of available wavelengths for laser communication.